1. Field of the Invention
This invention relates to a semiconductor device having a gate electrode.
2. Description of the Prior Art
A prior art semiconductor device comprises a semiconductor body having a P type anode layer, an N type base layer superposed thereon, a P type base layer superposed thereon, an N type cathode layer superposed thereon and an N type auxiliary region formed in the P base layer and being in close vicinity to the cathode layer. An auxiliary electrode, namely a pilot ring gate electrode formed on the auxiliary region surrounds a cathode electrode in contact with the cathode layer. The width of a corridor between the cathode electrode and the auxiliary electrode is constant on the whole circle.
But, because of forming a gate electrode on a peripheral zone which is located between the auxiliary electrode and the edge of the semiconductor body, an enlarged peripheral zone, namely a gate peripheral zone is formed. It is generally desired to obtain the largest current in the main thyristor, if possible. Therefore, when the disc wafer is utilized for the device, the cathode layer is substantially formed in a circular form. But, because of forming the gate electrode on the gate peripheral zone, the cathode layer is so formed as to have a cut off portion, and the form of the cathode electrode is similar to that of the cathode layer. The form of the auxiliary electrode is similar to that of the edge of the cathode electrode also, for the width of the corridor between the auxiliary electrode and the cathode electrode is constant.
On supplying a signal to the gate electrode, an initial turn-on current of the semiconductor device occurs on the auxiliary region or the neighborhood thereof. As the gate peripheral zone is larger than the other peripheral zone, a larger current flows into the auxiliary region or the neighborhood thereof. The current produced on the initial turn-on flows through the auxiliary region and the auxiliary electrode formed thereon and the corridor of the P base layer into the cathode layer. As a result, a main thyristor comprising the anode layer, the N base layer, the P base layer and the cathode layer is turned on. But, because of a lack of uniform width of the corridor and because the alumunum layer of the auxiliary electrode has a sheet resistance, the current does not flow uniformly from the auxiliary electrode to the cathode layer through the P base layer. The current produced in the enlarged peripheral zone is larger than the other current. The larger current flows through the auxiliary electrode adjacent to the enlarged peripheral zone into the cathode layer adjacent thereto. Therefore a current concentration is apt to occur on the zone adjacent to the enlarged peripheral zone. The semiconductor device would be destroyed by the current concentration.
In the case that a larger disc silicon wafer is utilized for maintaining a larger current therein, the current concentration would further easily occur in a limited zone of the wafer, that is to say the gate peripheral zone thereof. As the excessive current flows in the limited zone, the limited zone including an PN junction does not function as a PN junction, but as a resistance. As the current concentration is apt to occur in the semiconductor device when a surge current flows therein, the semiconductor device would be permanently destroyed.